Add include for missing ucontext64_t type

[tonyarnold]

📜 Description

Compiling main under Xcode 16.0 beta 5 produces the following error:

Missing '#include <sys/_types/_ucontext64.h>'; 'ucontext64_t' must be declared before it is used

This PR adds the aforementioned include.

💡 Motivation and Context

It allows users to continue building their apps that use Sentry under Xcode 16.0 beta 5.

💚 How did you test it?

I compiled under Xcode 16.0 beta 5 on an arm64 Mac.

📝 Checklist

You have to check all boxes before merging:

• I reviewed the submitted code.
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[brustolin]

Thanks @tonyarnold for the help.
I did reproduce this in here.

We just need a changeling entry, which I will add.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.456%. Comparing base (2911760) to head (be6d7c4).
Report is 2 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #4244       +/-   ##
=============================================
- Coverage   91.473%   91.456%   -0.018%     
=============================================
  Files          612       612               
  Lines        49341     49358       +17     
  Branches     17843     17860       +17     
=============================================
+ Hits         45134     45141        +7     
- Misses        4115      4123        +8     
- Partials        92        94        +2     

Files	Coverage Δ
...yCrash/Recording/Tools/SentryCrashMachineContext.c	68.750% <ø> (ø)

... and 14 files with indirect coverage changes

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